The Sudan Doctors' Committee, a group linked to the opposition, said a doctor and a child were killed by gunshot wounds to the head during the violence.

"One doctor and a child (have been) killed in today's demonstrations," the doctors' committee - part of a protest movement spearheaded by the Sudanese Professionals Association (SPA) - said in a statement.

The deaths were also confirmed to the AFP news agency by the relatives of the two victims, after protestors tried to march on the presidential palace, demanding the resignation of President Omar al-Bashir.

Protesters chanting "freedom, peace, justice" gathered in central Khartoum and began their march but riot police quickly confronted them with tear gas, witnesses told AFP.

Demonstrations also spread to other cities and towns, including the Red Sea city of Port Sudan, the provincial town of Gadaref and in the agricultural hub of Atbara, where the first protest broke out in December after a government decision to raise bread prices.

The protests have since escalated into broader demonstrations against Bashir's three decades of iron-fisted rule that have triggered deadly clashes with the security forces.

Officials say at least 24 people have died, but human rights groups have reported a higher toll.

Al Jazeera's Mohammed Vall, reporting from Khartoum, said these "demonstrations are considered the longest wave of protests against the government since the country's independence".

Before the protests, AFP reported that one of their journalists saw security personnel, many in plain clothes, stationed across the downtown area of Khartoum and along the expected route of Thursday's march.

Several army vehicles, mounted with machine guns, were stationed outside the palace.

Rising cost of essentials

Little traffic was seen at what is usually the height of morning rush hour.

The SPA - a trade union representing doctors, teachers and engineers among others - has stepped into the vacuum created by the arrest of many opposition leaders.

Mohammed Yousef, a spokesman for the association, said the protesters were prepared to continue to press their grievances while remaining patient and wise.

"The people of Sudan are known for being particularly determined, stubborn, and for playing the long game. They are not hot-headed, nor do they despair easily," he said.

Despite the crackdown, the movement has grown to become the biggest threat to Bashir's rule since he took power in 1989.

The protesters accuse Bashir's government of mismanagement of key sectors of the economy and of pouring funds into a military response Sudan can ill afford.

Sudan has suffered from a chronic shortage of foreign currency since South Sudan broke away in 2011, taking with it the lion's share of oil revenues.

UN expresses concern

"A repressive response can only worsen grievances," she warned, calling on Sudan's government to protect the protesters' right to freedom of expression and peaceful assembly, regardless of their political affiliations.

"The government needs to ensure that security forces handle protests in line with the country's international human rights obligations by facilitating and protecting the right to peaceful assembly," said Bachelet.